Vertical guides called masts are attached to motor driven vehicles, generally called tractors or trucks, for raising and lowering loads on a carriage slideable along the mast. Where the carriage mounts fork tines (“forks”), the trucks mounting the masts are known as forklifts. The term forklift has come to be used generically without regard to whether the carriage incorporates forks or not, and is so used herein. Extensible mast structures in which one mast member slides along another mast member allow a load carriage slideably mounted on the highest reaching mast member to be lifted higher than when the load carriage is mounted on a non-extensible mast. Ram drives alone or in combination with various systems of chains guided around pulleys or sprockets usually provide the motive power to lift the extensible masts. The extensible masts and the chains used to extend them are heavy. Forklifts for industrial use in warehouses and other fixed locations were developed many years ago that possess the ability to lift carriages and the loads on them without having to lift an extensible mast member. This ability is called “free lift.” The “free” in “free lift” means the carriage can be lifted without having to lift the extensible masts and associated chain and guide structures, saving time, gaining job efficiency and reducing wear and tear on the machine. In addition, by not having to lift extensible mast members to lift the load carriage, the forklift can work in areas of limited overhead clearance.
Some forklifts, called “truck mounted forklifts” are constructed to “piggyback” on the rear of a hauling vehicle such as a trailer pulled by a truck (as opposed to being driven onto a trailer and carried atop the trailer). The piggybacked forklift can be transported to a site, dismounted, and used to lift a load off the hauling vehicle and place the load at the site, or pick up a load at the site and place it on the hauling vehicle, or otherwise move loads at the site. In order to provide for piggybacking a truck mounted forklift, forklift receivers are constructed on the rear of a hauling vehicle for receiving the forks of the carrier. In order to piggyback itself onto a receiver fitted hauling vehicle, the truck mountable forklift elevates a fork carriage to a level of the fork receivers on a hauling vehicle, advances the forks into the receivers, and then operates the forklift as if to lower the carriage. However, since the elevated forks are fixed in the receivers, rather than the carriage descending, the rest of the forklift moves upwardly on the carriage, raising itself off the ground, allowing the forklift to be transported on the hauling vehicle. This lifting of the forklift by driving the forklift up the fixed fork carriage is called “negative lift.” Negative lift requires an ability either to positively pull or push the fork carriage down.
Examples of a mast lift system employing rams and chains to drive the carriage up or down and give negative lift capability are U.S. Pat. No. 4,921,075 and U.S. Pat. No. 5,328,321. The former patent describes a two stage mast system, and the latter a three stage mast system. In both, an arrangement of chains and pulleys positively lifts and lowers a carriage responsive to a double acting hydraulic ram mounted to a stationary mast section lifting or lowering an extensible mast slideable on the stationary mast. The system exemplified by these patents requires that the extensible mast member or members be raised in order to raise the fork carriage to an elevation allowing the forks to be inserted into fork receivers on a hauling vehicle. Because the fork carriage cannot be raised without raising the extensible mast members, this system does not have free lift. Further, because the extensible mast sections must be extended to lift the forks on the fork carriage, the height of the forklift is increased during operation, limiting usefulness of the forklift in areas with limited overhead clearance.
On the other hand, the usual free lift system is not capable of negative lift. The usual manner of free lifting a fork carriage is by indirection, using a chain fixed to a stationary location, passed over a wheel or sprocket, and attached at the other end to the carriage. The axle of the wheel or sprocket is borne on a clevis or other mounting device at the end of a rod of an upright hydraulic actuator. When the rod elevates as fluid pressure is applied to the actuator, the wheel or sprocket is raised, lifting the chain and pulling the carriage upwardly without lifting the extensible masts. The chain prevents the carriage from moving downward while the rod of the hydraulic actuator is extended. However, only gravity restrains upward movement of the carriage. If the rod of the hydraulic actuator for the carriage is extended for placement of the forks in receivers in a hauling vehicle, operation of the mast lift actuators to lower the extensible mast members will not prevent the carriage from sliding up an inner mast section on which the carriage is supported. Thus the typical free lift system cannot be used for piggyback transportation.
U.S. Pat. No. 7,255,202 describes an effort to accomplish both free lift and negative lift with a carriage slideable on an innermost extensible mast of a forklift truck. Free lift is obtained in the usual way with a chain fixed at one end and with the other end attaching to the carriage and passing over a wheel mounted on a rod of a ram (hydraulic actuator) mounted right side up on a lower cross member of the innermost extensible mast member. The carriage is prevented from sliding up the innermost mast section by a hydraulic actuator mounted upside down to an upper cross member of the innermost extensible mast member, with a chain fixed at one end and with the other end attached to the carriage and passing under a wheel carried by the rod of the upside down actuator. When the rod of the upside down actuator is raised, the pulleyed chain pulls the carriage down, and so long as held down by the raised rod of the upside down actuator, the carriage is preventing from sliding up the innermost mast section when forks on the carriage are held secured at an elevated position. The two carriage actuators interchange the hydraulic fluid actuating them, so that fluid is pumped from one to the other to retract the rod of the one and extend the rod of the other, and vice versa.